Laminated prop in paper - Page 4

**pahorace** · #31 03-09-2009, 11:23 AM

Thread of high quality and precision. Do not miss it!
The scale 1/48, very important to complete.
Thanks for the effort and for sharing with us all.

Orazio

**Leif Ohlsson** · #32 03-09-2009, 12:07 PM

Thanks Orazio & Armorman, for your appreciation!

To set the record straight, the museum showing the aerobatic aircraft BA 122 is the Letecke Museum, Kbely near Prague.

And the engine is not the 100-110 hp Shvetsov M-11, but Avia Rk-17, delivering no less than 430 hp.

Small wonder that it would need a really effective air-pushing devise up front!

For looking at all the sometimes beautifully restored aircraft in the museum, use the link supplied above.

**Leif Ohlsson** · #33 03-09-2009, 12:16 PM

I noticed that Alin Osarik's beautiful & advanced Si-204 (announced in this thread) contained a pilot figure.

So I sent him a small request - would it be possible, sometime in the future, when all the work with the Si-204 was done, to slice this pilot in 0.2mm thick slices at different scales?

If so, it would be possible to make a laminated pilot figure, like the propellers here.

Two hours later the attached arrived!

Things sometimes happen faster than you could ever imagine.

Now I am eagerly waiting for a version with vertical slices instead, since I believe that would make more sense, and model the face better. We'll see.

Anyway, thanks to Alin, the ball has been set in motion.

Leif

**pahorace** · #34 03-09-2009, 12:44 PM

Quote:

Originally Posted by Leif Ohlsson

..... cut .... cut ...
Two hours later the attached arrived!

Things sometimes happen faster than you could ever imagine.

Now I am eagerly waiting for a version with vertical slices instead, since I believe that would make more sense, and model the face better. We'll see.

Anyway, thanks to Alin, the ball has been set in motion.

Leif

This, I believe, would be a wonderful event, a step forward in the technology applied to paper-model that can be exploited for many other things (I think of warheads such as the propeller and other rounded shapes).

Orazio

**milenio3** · #35 03-09-2009, 05:57 PM

Leif, the tutorial is a top notch!
Thank you very much!

**CharlieC** · #36 03-09-2009, 06:35 PM

I believe this method will have applications far beyond the original wooden propellers. IMHO this is a breakthrough in cardmodeling.

I can see that this technique could be applied to model cast and 3 dimensional parts in AFV models. I guess the technique could be extended to model cast turrets and other parts which are difficult to model in card.

I've attached some possible targets for this method.

T-18 cupola
BT-7 mantlet and periscope mounts.
M4A3 Sherman turret

Regards,

Charlie

**Barry** · #37 03-09-2009, 08:26 PM

I remember seeing this as a plan for a Morris Minor body consisting of dozens of thin balsa strips for those like me who could not carve wood it was about 55 yrs ago oh dear. I suppose with advent of crazy glue it's feasable again

**Leif Ohlsson** · #38 03-27-2009, 06:28 AM

Below you will see what I think is a very pretty propeller. It is made for the Po-2, and the making of it has occupied me for the last couple of weeks.

It is made very closely to the drawings of the original aircraft that were unearthed in this thread, some of them available as a download from the aviation downloads section.

The prop depicts a seven-layered original, and in the 1/16 scale version shown here it contains 42 parts (plus various hub parts, etc.). I would estimate that there are about 200 vector points in it, each carefully adjusted in a long series of iterations to achieve as smooth and realistic result as possible.

It is just one step away from being made into a kit, which would be the lesser amount of work.

And yet I am going to scrap it.

Why? Because it is totally unrealistic from an aerodynamic point of view, and I could beat myself up for believing that the Russian drawings were accurate. I now think they are a product of the draughtsman's imagination - well-meaning no doubt, but still...

In what follows, I will use an analysis of this failed attempt to build some solid foundation for a truer scale prop for the Po-2 (and similar aircraft). I hope it might prove educational for others as well.

Leif

**Leif Ohlsson** · #39 03-27-2009, 06:30 AM

Looking at the profile sections of the prop as it now is, you will soon see why it isn't aerodynamically feasible. In the first picture below, I have added the profiles used. Can you see how similar the angles of attack of the profiles are throughout the prop length?

This should not be so. The prop is a wing, and at each point along its lenth the wing profile should meet the airstream at the optimal angle. The resultant airstream hits the prop at widely differing angles along the length of the blade, since it arises from a combination of airspeed and rotational speed.

The airspeed is constant, namely the speed the prop is optimized for (probably the cruise speed). The rotational speed is proportional to the distance from the hub, and thus varies greatly.

How come I didn't react to this fact? Let's go to image nr. 2 below. This is the original Russian drawing, and as you can see the different profiles are drawn in great detail, seemingly authoritatively. This is what I trusted.

However, there were reasons to be suspicious even from the start. In image nr. 3 I have drawn what I call "profile boxes" for each profile. The base of each box is the width of the propeller at that section, as seen from the front (right half of the figure). The height of the box is the thickness of the propeller at that section, as measured from the side view (left half of the figure).

As you can see the profiles drawn do not conform to these boxes, which they must do, if they are to be physically possible at all.

So what I did (see image nr. 4) was to accept the profiles as drawn, but change their size and incidence (angle of attack), to conform to these profile boxes.

In addition, I added a new section at the very top, and one close to the hub. The profile of the latter was a real headache to arrive at. It took me any number of complicated iterations, but more of this at some later point.

We can now return to the prop as drawn, and mark the actual angles of attack (image nr 5). This is how the prop is now. In the next post, I'll analyze how it ought to be.

Leif

**Leif Ohlsson** · #40 03-27-2009, 06:32 AM

In the first image below I have added a few basic calculation to the right of the drawing. They are measurements of the distance of the different sections from the hub.

I took profile nr 3 as my point of departure, since at 47 percent of the radius it is close to the middle of the prop. If this is taken as having a correct angle of attack, what will the appropriate angles of attack for all other sections be?

You calculate this by figuring out what the components of the resultant airstream over each prop section are. For profile nr. 3 I have added them in the shape of a red box. The height of the box is the aispeed of the aircraft, which is constant at the speed the prop is optimized for. The base of the box is the rotational speed, which varies proportionally to the distance from the hub.

In image nr. 2 I have made these changes of rotational speed (base of box), and added them as red boxes and appropriate angles of attacks at all the stations. As you can see, the top profiles should have increasingly lower angles, and the lower profiles much higher than at present.

The bottom profile indicates an impossibly high angle of attack, which leads me to believe that the angle of the profile nr 3, which we took as our point of departure, is off as well. (Or, if profile nr 3 really is correct, it would mean that the base section has a very peculiar profile. It will be most interesting to see what comes out of the continued exercise.)

So, what to do now? I would like to get down to some serious business and calculate some real angles, based on known and/or assumed basic facts of the full scale aircraft.

For this exercise I will start with the stated cruise speed of 110 km/h and an assumed engine speed of 1600 rpms. Any points of view on this? I am guessing the rpm, and would appreciate some authoritative input here.

Once decided upon, these figures should enable a correct prop design, since the diameter of the full scale prop, and frontal profile, is known.

Can you see some other consequences of this way of doing it? For one thing, I am pretty sure that the side profile (left part of the drawing) will not stay the same! Perhaps we will arrive at something like the Camel prop's side profile...

Leif